An optical amplifier using an erbium (Er) doped fiber (EDF) is known as an optical amplifier using a rare-earth-doped amplifying medium. The EDF amplifies an input light with a gain according to the level of a pump light of 0.98 μm band to 1.48 μm band. A laser diode (LD) (semiconductor laser) is used as a pump source.
The LD oscillates to emit a light by a cavity structure. Recently, the oscillation wavelength of an LD is set by an external cavity structure in which an external reflector that reflects only a given wavelength is connected to an output fiber of the LD. The wavelength and the intensity of the light output from the LD are determined mainly based on the gain due to an externally-provided current and the cavity length.
The output power of the LD may become unstable if the oscillation mode of the LD fluctuates due to external reflection and/or polarization state, particularly when the driving power of the LD is low. The cycle of the fluctuation when the LD is unstable is highly dependent on the external environment, and thus indefinite. The optical amplifier using the EDF performs feedback control of the LD of the pump source based on a monitored value of the output from the optical amplifier. It is also known to superimpose a high frequency to improve the instability of the LD (see, for example, Japanese Laid-Open Patent Publication Nos. H8-32162 and H8-204267).
However, in the conventional technologies described above, the feedback control of the LD based on the value output from the amplifying medium cannot be on time if the output power of the LD fluctuates faster than the response speed of the amplifying medium, and the power of the light output from the amplifying medium disadvantageously fluctuates and cannot be stabilized. Further, the superimposed high-frequency component accumulates and affects the transmission quality, particularly when multiple optical amplifiers are connected.